1. Field of the Invention
The present invention relates to a method of feedback-controlling the idling speed of an internal combustion engine, and more particularly to an internal combustion engine idling speed feedback control method in which the engine operation is smoothly shifted from a deceleration state in which the throttle valve is fully closed to an engine operating condition controlled by an idling speed feedback control, thereby improving the engine operating performance.
2. Description of the Prior Art
Hitherto, an idling speed feedback control method has been used in which a target idling speed is set in accordance with load conditions of an engine, and the difference between the target idling speed and an actual engine speed is detected. The amount of auxiliary air to be sucked into the engine is then adjusted in accordance with the magnitude of the difference such that the difference becomes zero, thereby maintaining the engine speed at the target idling speed (e.g., Japanese Patent Laid-Open No. 98,628/80).
When the engine operating state is shifted from a deceleration state wherein the throttle valve is fully closed to an idling state, if the auxiliary air amount control is commenced by the above-described feedback control at the time when the engine speed becomes lower than the target idling speed, the engine speed drops below the target idling speed due to a delay in the feedback control. Thus, the engine may stall when there is a sudden drop in speed. In order to overcome this disadvantage, a method has been proposed in which the amount of auxiliary air required for maintaining the engine speed at the target idling speed is supplied to the engine during deceleration, before the feedback control is commenced.
In general, when the deceleration is effected, wherein the throttle valve is fully closed, the charging efficiency of the engine is small, and the charging efficiency decreases as the rate of deceleration of the engine increases. This fact becomes more conspicuous as the volume of the chamber of the intake manifold is increased in order to increase the charging efficiency in a heavy-load operation, such as an internal combustion engine equipped with, for example, an electronic fuel injection device. For this reason, even if an amount of air corresponding to the target idling speed is supplied during deceleration as described above, when the rate of deceleration is large, since the charging efficiency becomes smaller, the engine speed may disadvantageously overshoot below the target idling speed by a large margin when the control is shifted to the idling speed feedback control.